The modern era of catalytic reforming for high-octane gasoline began in 1949 with the introduction of platinum-containing catalysts, which swept the industry during the 1950's and continue to form the basis of modern reforming catalysts and processes. Fluidized-bed catalytic reforming, often characterized as fluid hydroforming, was known from the early days of catalytic reforming. This technology failed to play a prominent part in the commercial arena and has been in decline, even though it is based on the attractive concept of flexibility in operating conditions and ready removal and regeneration of catalyst. Problems relating to temperature control in relation to the endothermic heat of reaction, stripping, regenerating and returning catalyst in different atmospheres, and the recovery of catalyst fines are believed to be factors in the lack of widespread success. More recently, moving-bed catalytic reforming units associated with continuous catalyst regeneration have addressed these problems and dominated new-unit construction.
Catalytic reforming involves a number of competing processes or reaction sequences. These include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, hydrocracking of paraffins to light products boiling outside the gasoline range, dealkylation of alkylbenzenes and isomerization of paraffins. Some of the reactions occurring during reforming, such as hydrocracking which produces light paraffin gases, have a deleterious effect on the yield of products boiling in the gasoline range. Process improvements in catalytic reforming thus are targeted toward enhancing those reactions effecting a higher yield of the gasoline fraction at a given octane number.
Programs to improve catalytic-reforming performance of are being stimulated by the reformulation of gasoline, following upon widespread removal of lead antiknock additive, in order to reduce harmful vehicle emissions. Gasoline-upgrading processes such as catalytic reforming must operate at ever-higher efficiency with greater flexibility in order to meet these changing requirements. The lowering of operating pressure, maintenance of catalyst selectivity, and attention to reaction-temperature optimization are important parameters in achieving improvements in the reforming process. Fluidized-bed reforming offers the potential for exploiting these parameters.
U.S. Pat. No. 3,033,780 (McGrath et al.) teaches fluid hydroforming of a light hydrocarbon oil to obtain a high anti-knock motor fuel. The hydrocarbon oil and a hydrogen-containing gas are supplied separately to a reaction zone, with the gas being heated to a higher temperature than the oil to supply a portion of the endothermic heat of reaction. Catalyst particles are withdrawn, stripped, regenerated and recycled. Reaction products exchange heat with the feed and are withdrawn and separated.
U.S. Pat. No. 3,776,838 (Youngblood et al.) discloses catalytic cracking of naphtha with a zeolite cracking catalyst in successive elongated reaction zones followed by a catalyst phase in a reactor. A fraction boiling between 100.degree. and 450.degree. F. is recovered from the reaction mixture from the first elongated zone and introduced along with zeolite catalyst to the second elongated zone.
U.S. Pat. No. 5,030,782 (Harandi et al.) teaches a two-stage conversion process in which aliphatics are cracked and dehydrogenated in a fluid bed to yield an intermediate product which is processed with an aromatization catalyst. C.sub.4 -olefins are formed in the cracking/dehydrogenation zone, reacting in the aromatization zone to provide a portion of the endothermic heat of reaction.
The problem facing workers in the art is to find modifications to the known fluidized-bed technology which would render it commercially attractive in today's environment of alternative catalytic reforming processes, gasoline specifications and aromatics needs.